The present invention relates to control circuits for controlling operation of an electrical load, such as an electromagnetic brake, using low power safety relays.
Elevator systems typically make use of an electromagnetic brake to hold the elevator car in position when the car is stopped, and to provide emergency braking in the event of an over-speed or over-acceleration condition, a controller circuitry failure, or loss of power. A typical elevator system may include a hoist motor having a drive shaft for rotating a sheave; and a cable, belt, or rope that extends over the sheave and between the elevator car and a counterweight. An elevator brake is mounted on or adjacent the motor drive shaft or the sheave to control rotation of the shaft and sheave.
The elevator brake is typically an electromagnetic brake that is spring biased so that a brake pad or shoe is pressed against a braking surface when there is no current flowing through the brake coil. The brake is released, allowing rotation of the drive shaft and movement of the elevator car, when a sufficient current flows through the brake coil to move the brake pad or shoe out of contact with the braking surface. The current required to move the pad or shoe so that the brake is released is referred to as the pick current or lift current. Once the shaft is rotating and the car is moving, the amount of current required to maintain the brake pad or shoe out of contact with the braking surface (referred to as the hold current) is less then the pick or lift current. When the power applied to the brake coil is turned off, the electromagnetic field collapses, and the brake pad or shoe is moved by the spring by its back into contact with the braking surface.
The hoist drive motor of an elevator system may be an induction motor driven by an inverter that converts DC voltage to an AC drive. A drive control may control operation of both the drive motor and a brake control circuit that energizes and denergizes the electromagnetic elevator brake. An elevator control controls the starting, running, and stopping of elevator cars, and provides signals to the drive control to coordinate operation of the drive motor and the elevator brake.
The brake circuit may also be powered by DC power, and includes relays or semiconductor switches to control the delivery of the DC power to the electromagnetic brake. The relays or semiconductor switches must be capable of energizing and denergizing the brake in response to control signals from the drive control, and also must be capable of denergizing the brake to stop the elevator car when a potentially unsafe condition occurs.
Elevator systems typically make use of a safety chain that includes hoistway door contacts on each hoistway door that are connected in series with the power supply and drive motor of the elevator. A top of car inspection switch and a pit emergency stop switch may also be connected in the safety chain. The opening of a single hoistway door contact will break a connection between the power supply and the drive motor and the elevator brake, and prevent movement of the car as long as the hoistway door is open.
During a normal stop at a floor, the hoistway door and the elevator car doors will open for a short period of time to allow passengers to enter or exit the elevator car. The doors will then close again, and the safety chain is closed so that elevator brake is released and the car can move in the hoistway to its next stop.
If a hoistway door is opened manually when a car is not in position adjacent that hoistway door (i.e. an abnormal opening of the hoistway door has occurred), the safety system will prevent normal operation of the elevator until a latch condition caused by the abnormal opening is safely cleared. The safety system operates on an assumption that whenever an abnormal opening of a hoistway door occurs, a person or persons could have entered the elevator hositway. To prevent possible injury of authorized or unauthorized personnel that may have entered the hoistway while the hoistway door was open, the elevator system enters a shutdown condition that prevents elevator motion until a special sequence is followed.
Currently, elevator brakes are typically controlled by large relay contacts or expensive semiconductor switches. The entire energy flow from the power source to the brake coil is switched by these relay contacts or switches. As the requirements of elevator systems have evolved, larger more powerful electromagnetic brakes are required. The high inductance and greater power demands of these larger brakes has required safety relay contacts that are capable of switching high currents. This can result in shorter operating life of the safety relays. In addition, the properties of the safety relays required for larger electromagnetic brakes has precluded the mounting of the safety relays on a common circuit board with the other circuitry of the brake control, and has complicated wiring because the safety relays are connected in series with the brake.